Technical Field
The present disclosure refers to the field of the electric motor control, and particularly to three-phase electric motors.
Discussion of the Related Art
In the area of the three-phase electric motor control, the adopted techniques typically require an accurate measurement of the angular position of the rotor and of the motor phase currents.
In some control techniques, the rotor angular position is measured through suitable position sensors directly attached to the rotor. In order to ensure a high precision, position sensors, such as for example incremental encoders and resolvers, are used, while for containing the costs and in some applications wherein sensors of position operate in harsh environments having characteristics such to compromise their operation, are used low resolution sensors such as for example Hall sensors. It is to be noted that Hall sensors ensure a precision which is sufficient just for some applications.
Therefore, control techniques, known as sensorless, were developed which indirectly determine the rotor angular position without position sensors.
A sensorless control technique provide to find the rotor position by determining the zero-crossings of the induced electromotive force or back-EMF. Indeed, the zero-crossings of the back-EMF correspond to known angular positions of the rotor having a resolution of 60 electrical degrees. By means of an outer circuit detecting such zero-crossings of the back-EMF, the rotor angular position is measured with a good precision and it is reconstructed in the intermediate time instants between two consecutive zero-crossings, through a simple linear interpolation.
Despite this technique is advantageous for various aspects, it is only used for controls wherein just two current phases in turn are excited since the back-EMF measurement is performed in the non-excited phase of the motor.
Therefore, such techniques are used in case of a trapezoidal trend of the back-EMF, such as for example the one represented in FIG. 1, referred to a brushless three-phase motor (BLDC Pittman 3441S001-R3). FIG. 1 also illustrates the trend of signals generated by Hall sensors.
Such control technique should be avoided if the aim is to induce, in the motor phases, drive electric currents having a sinusoidal trend. Indeed, in such case, it is not possible to measure the back-EFM, since all the phases are simultaneously excited.
Some approaches providing to simultaneously shutting off the excitation to the phase for a time interval sufficient to perform the measurement, are known. However, such approaches introduce a distortion of the currents which cannot be accepted for some applications. Further, such approaches require a computational load because, besides turning off and on the excitation of a phase in determined instants, it is necessary the use of an algorithm which anticipates the zero-crossing of the back-EMF in order to act in time.
Therefore, such control modes are hardly applicable in the sinusoidal control techniques or in the field-oriented control techniques, wherein the motor is simultaneously driven in all the three phases.
Other control techniques provide to measure the phase currents of the motor and to reconstruct the rotor angular position by means of complicated state observers or state sensing circuits. However, the state sensing circuits require control algorithms having a high computational complexity entailing a limitation due to the excessive run time.